Slow-release fertilizer is a type of plant fertilizer that delivers plant nutrients over a longer period of time in comparison with fast-release fertilizer. The quickness of release refers to the availability of nitrogen that can be used by plants. Slow-release fertilizer contains nitrogen in forms that are not soluble in water and must be broken down by soil organisms before they can be taken up by plants. Consequently, slow-release fertilizers have their own advantages and disadvantages. Slow-release fertilizer can be a synthetic chemical preparation, or be derived from organic material such as plants or animal byproducts.
Fertilizers usually contain some mix of the three elements most important for plant growth, which are nitrogen, potassium, and phosphorous. Nitrogen is necessary for sturdy shoots, healthy leaves, and for making chlorophyll. Chlorophyll is the molecule that allows plants to convert energy and gives them their green color.
Slow-release fertilizer has certain advantages and disadvantages. One advantage is that nitrogen is released longer, so slow-release fertilizers provide the nutrient over a longer period of time. Although this means plants will not show a fast response to the fertilizer, it also reduces the chances of burning plants, which results from over-fertilizing. Another advantage that can save time and money is that less fertilizer is likely to be lost to rain or watering in the process known as leaching. While slow-release fertilizer tends to be more expensive, its longer period of availability means fewer applications will have to be used.
Organic slow-release fertilizers, made from plant and animal material, contain nitrogen and other plant nutrients in varying amounts. Ground hooves and horn contain a good deal of nitrogen that is released slowly. Other types of naturally-derived fertilizers include finished compost, seaweed meal, and certain animal manures. Unfinished compost and mulches can reduce the amount of available soil nitrogen to some extent. Chemical slow-release fertilizers usually contain synthetic preparations of urea, a nitrogen-containing substance produced by many animals.
The rate at which nitrogen becomes available from slow-release fertilizer depends on a few factors. As the action of soil organisms is needed to make it effective, it is best applied in the spring and summer when these microorganisms are most active. Healthy soils contain these organisms naturally. As soil requirements for nitrogen vary, slow-release fertilizer can be purchased with differing amounts of this nutrient. Soil tests can determine the amount of nitrogen needed, and this number can be compared to the level listed on many of the commercially available fertilizers.